Electric fuses



Nov. 4, 1958 K. DANNENBERG 2,859,308

ELECTRIC FUSES Original Filed Sept. 1, 1953 i IIII 5; glllm INVENTOR KURT DANNENBERG BY a w ATTORNEY United States. Patent Ofiice 2,859,308 Patented Nov. 4, 1958 ELECTRIC FUSES Kurt Dannenberg, London, England, assignor, by mesne assignments, to I-T-E Circuit Breaker Company, Philadelphia, Pa., a corporation of Pennsylvania Original application September 1, 1953, Serial No. 377,874, now Patent No. 2,800,554, dated July 23, 1957'. Divided and this application December 21, 1956, Serial No. 629,886

Claims priority, application Great Britain July 23, 1953 4 Claims. (Cl. 200-131) The present invention relates der-filled electric fuse.

7 Such fuses are designed to operate very rapidly-under short circuit conditions due to the interaction of the powder filling with the metal sible element. When such fuses are also to be used for overload protection, for example for protecting cables against overheating, for protecting lighting installations and for protection against high resistance earth faults, it is also desirable on the other hand that the fusing factor shall be as small as possible. The fusing factor is defined as the ratio of the minimum fusing current to the continuous current-carrying capacity of the fuse. If it is desired that the fusing factor as above defined, of such fuses, shall be substantially less than 2 and, in particular, less than 1.5, considerable difiiculty has heretofore been met in satisfactorily meeting this requirement, and the object of the present invention is to provide a construction of fuse permitting comparatively low values of the fusing factor, down to about 1.25 or below, to be reliably obtained.

In co-pending application Serial No. 377,874, now Patent 2,800,554, of which the present application is a division, a current limiting powder filled fuse is described comprising conductive .connector blocks disposed at opposite ends of the fuse, conductive connectors respectively connected to said conductive blocks for connecting said fuse in a circuit, a plurality of identical conductive fusible elongated elements extending between and connected at the respective ends thereof separately to said conductive connector blocks, said elements being in the form of thin strips of the same length and of the same cross-section in corresponding portions of the length thereof to ensure even sharing of the load .current between said elements and simultaneous and identical operation on short circuit faults, an outer casing or cartridge, a filling of arc-quenching powder within said casing and surrounding all said strips in heat transferring relation thereto, all of said strip and said powder filling being selected to operate with current limiting action upon operation of said fuse, at least a selected one of said strip carrying a mass of low to a current-limiting powmelting point metal in conductive contact with and sur-.

rounding a short portion of the length of said selected strip, said mass of metal being omitted from at least another of said strips to impart a different fusing characteristic under overload current conditions to said selected strip from said other strips to cause preferential fusing of said selected strip under such overload current conditions.

The present application is divided out of the aforesaid co-pending application and it has for its object to provide a device having the same operating properties wherein different elements of a plurality of parallel connected load carrying fusible elements are formed from different metals, one of said elements having a lower melting point than another of said elements and the cross-sectional areas of the respective elements being selected to maintain in said elements the same effective resistance to current flow in all of them.

vapour produced from the fu- 1 By this construction one or more of the elements has different fusing characteristics than one or more other elements such that the former element or elements or at least a certain part or parts thereof melt at a lower current or more rapidly under moderate overloads; all the elements however are heated to approximately the same extent and in general they all carry approximately the same proportion of the total current and moreover they all fuse simultaneously under short circuit conditions.

By this construction a fuse can be produced having a comparatively low fusing factor (as above defined), substantially below 1.5-down to about 1.25 or even less. In operation under overload conditions the overload current is initially distributed uniformly between the several ele ments, but that element or those elements which is/are adapted to fuse at a lower current or more rapidly, is/ are caused to melt and break the current path through it or them, with the result that the current previously carried by it or them is transferred to the other element or elements and the latter now interrupt the circuit current by fusing in the normal manner by virtue of the additional current .carried by it or them.

In carrying the present invention into effect one group of elements (consisting of one or more but not all of the elements) is formed of material of a lower melting point metal than the other element or group of elements, but all the elements are proportioned so that substantially the same amount of heat is generated by the current traversing them or that they all attain the same working temperature; in general this implies that the two groups have different cross-sectional areas or widths so that all the elements are of the same resistance. All such elements fuse simultaneously under short circuit but under moderate overloads the first group fuses before the other group to allow for a similar operation to that already described. For example the first group may be formed of tin strip and the other group of silver strip, the former group being of greater width than the latter group due to the greater specific resistance of tin than of silver.

It is advantageous to use silver for the fusible element or elements of higher melting point owing to its low specific resistance and to the favourable action obtained between the silver vapour and the powder filling upon fusing.

The figure of the drawing shows the fuse of the invention in elevation with the casing broken away.

A preferred embodiment of the present invention is illustrated in the drawing and comprises three fusible strips incorporated in a fuse structure of the character shown in Fig. 1 of the drawings of the aforesaid co-pending application and comprising an outer cartridge 7 and end caps 9 having metal connector blocks 10 to which the fuse strips shown on the accompanying drawings are electrically attached. The cartridge 7 is provided with a filling of arc-quenching powder as is well-known in the art, for the purpose of obtaining a high resistance break between the end caps on interruption of the circuit by the fuse by interaction between the volatilised metal and the powder filling, as will be understood by those versed in the art, to provide a powder filled fuse having a current limiting action.

The arrangement shown on the drawing includes an alternative provision to ensure a low fusing factor and for this purpose one element, for example the fusible element 12 in the example shown which utilises three elements in all, is arranged to have different fusing characteristics under moderate overload conditions from the other element or elements such as 11 and 13.

As shown on the accompanying drawing three fusible elements 11, 12 and 13 are provided in which one or more of the elements, for example the element 12, is formed by a different material from the other elements 11 and 13. For example the elements 11 and 13 can be formed of silver strip while the element 12 may be formed of tin. The latter is however of larger cross section so that the effective resistance of all the strips is substantially the same. In this case reliance-is placed on the lower melting pointioflthetin strip 12 whichicauses said stripto'fuse before: the strips 11. and. 13 under overload conditions resulting in substantially a similar action to that already described, whereas under short circuit conditionsall; the elements fuse. substantially together in the normal manner applicable in. the case of powder filled fuses.

The centre parts. of. all the elements are of reduced widthas indicated at 15. As is well-known this type of fuse is capable of: actingas a current limiting fuse ensuring, under short circuit conditions, interruptionof the currentin the course of the first half cycle of the fault current.. An importantfeature ofthe invention is that holes 16 are provided near theends of the centre parts. This construction ensures optimum operation under short circuit conditions, in: particular by ensuring that fusing occurs. initially' at' the centre of each element and then spreads progressively towards the ends, thus ensuring a quick interruption of the flow of fault current but not,

however, in such. manner as might engender the production1of excessive voltages in-ithe-circuit.

The invention preferably also includes an indicator plunger 22, as in Fig. 1 of the said co-pending application,.which is adaptedto show the condition ofthe fuse. The plunger 22'is normally located in a. housing or recess 23 in' one end cap 9 and is urged outwardly by a spring 24 insaid housing.v The plunger is, however, held in its normal position by a length. of wire 25 for example of nickel. chrome alloy which. is anchored to the opposite end cap 9 and which includes a tension spring 26. When fusing of the fusible elements occurs current is. divertedinto the wire 25 which therefore also fuses and releases the plunger which moves outwardly under the action of the spring to give avisual indication.

What I claim is:

parallel with each other, different elementsbeing'formed from' different metals; a given one of'said elementshavl ing a lower melting point than another of said elements, the cross-sectional area. of said givenelement being greater than the cross-sectional area of said other element and being proportioned relativethereto substantially to maintain in said elements thevsame efiective resistance to current flow in the several elements.

2. A fuse as defined in .claim 1 wherein said other element is formed of silver and said given element is formed of tin and has a larger cross-sectional area than said silver element proportioned relative to said cross-sectional area of said silver element to maintain the same effective resistance as said silver element to current flow.

3. A current limiting fuse comprising a plurality of conductive fusible elements connected in parallel with each other, a given one of said elements being formed of a conductivematerial having a lower melting point than the material of another of said elements, the least current carrying cross-sectional area of said given element being greater than the least current carrying CIGSSrSfiCtiODHl area of said other element and being proportioned relative to said least cross-sectional area of said other element to provide substantially the same effective resistance to current flow in the two elements, whereby said given element will fuse first under limited overload current and all said elements will fuse substantially simultaneously under short circuit current.

4. A current limiting fuse as defined in claim 3 in which said cross-sectional area in each element is less along a central portion of the length of said element than in the adjacent end portions thereof, at least a part of the reduction of said area of said central portion in each element being provided by a hole through the material of said central portion.

References Cited in the file of this patent UNITED STATES PATENTS 2,800,554 Dannenberg et al July 23, 1957.

FOREIGN PATENTS 12,210 Great Britain May- 18, 1914 638,021 Germany Nov. 7, 1936 815,875 France Apr. 19, 1937 

